You can't change just one gear because the cluster gear is one-piece. An aftermarket "kit" would require both a custom cluster and 5th gear ($$$), plus the labor to install it and then you might as well do a rebuild while its apart...

If you have an R150 trans (V6), you can swap in an R154 (Supra) gear set. That is the only thing available that I know of and even that hardly seems worth it. And it changes more than just 5th gear too.

'97 4Runner, '06 F350, '86 4Runner, '05 WR450
http://home.4x4wire.com/erik